The game was never released because it was a test game to test out what the Amiga 500 could do, and if it could handle a Sonic-like game. Although the game was never released, somebody found the ADF (or rom) for the game: https://goo.gl/KZIdxN

An explanation as to why it might have less-than-original sprites is because the game was never officially released, and the disk image only has one level on it, so I'm under the impression it was not finished when it was discovered.